A light emitting diode (which will be referred to herein as an “LED chip”) is a semiconductor device that can radiate an emission in a bright color with high efficiency even though its size is small. The emission of an LED chip has an excellent monochromatic peak. To produce white light from LED chips, a conventional LED lamp arranges red, green and blue LED chips close to each other and gets the light rays in those three different colors diffused and mixed together. An LED lamp of this type, however, easily produces color unevenness because the LED chip of each color has an excellent monochromatic peak. That is to say, unless the light rays emitted from the respective LED chips are mixed together uniformly, color unevenness will be produced inevitably in the resultant white light. Thus, to overcome such a color unevenness problem, an LED lamp for producing white light by combining a blue LED chip and a yellow phosphor was developed (see Patent Documents Nos. 1 and 2, for example).
According to the technique disclosed in Patent Document No. 1, white light is produced by combining together the emission of a blue LED chip and the yellow emission of a yellow phosphor, which is produced when excited by the emission of the blue LED chip. That is to say, the white light can be produced by using just one type of LED chips. That is why the color unevenness problem, which arises when white light is produced by arranging multiple types of LED chips close together, is avoidable.
However, the luminous flux of a single LED chip is too low. Accordingly, to realize a luminous flux comparable to that of an incandescent lamp, a fluorescent lamp or any other general illumination used extensively today, an LED lamp preferably includes a plurality of LED chips that are arranged as an array. LED lamps of that type are disclosed in Patent Documents Nos. 3 and 4, for example.
Patent Document No. 5 discloses an LED lamp that can overcome the color unevenness problem of the bullet-shaped LED lamp disclosed in Patent Document No. 2. First, this color unevenness problem and a configuration for an LED lamp that is designed to overcome such a problem will be described.
The LED lamp with the bullet-shaped appearance as disclosed in Patent Document No. 2 has a configuration such as that illustrated in FIG. 1. As shown in FIG. 1, the bullet-shaped LED lamp 200 includes an LED chip 121, a bullet-shaped transparent enclosure 127 to cover the LED chip 121, and leads 122a and 122b to supply current to the LED chip 121. A cup reflector 121 for reflecting the emission of the LED chip 121 in the direction pointed by the arrow D is provided for the mount portion of the lead 122b on which the LED chip 121 is mounted. The LED chip 121 is encapsulated with a first resin portion 124, in which a phosphor 126 is dispersed and which is further encapsulated with a second resin portion 125. If the LED chip 121 emits a blue light ray, the phosphor 126 is excited by the blue light ray to produce a yellow light ray. As a result, the blue and yellow light rays are mixed together to produce white light.
However, the first resin portion 124 is formed by filling the cup reflector 123 with a resin to encapsulate the LED chip 121 and then curing the resin. Therefore, the first resin portion 124 easily has a rugged upper surface as shown in FIG. 2 on a larger scale. Accordingly, the thickness of the resin including the phosphor 126 loses its uniformity, thus making non-uniform the amounts of the phosphor 126 present along the optical paths E and F of multiple light rays going out of the LED chip 121 and passing through the first resin portion 124. As a result, unwanted color unevenness is produced.
To overcome such a problem, the LED lamp disclosed in Patent Document No. 5 is designed such that the reflective surface of a light reflecting member (i.e., a reflector) is spaced apart from the side surface of a resin portion in which a phosphor is dispersed.
Hereinafter, an exemplary LED lamp as disclosed in Patent Document No. 5 will be described with reference to FIGS. 3(a) and 3(b).
In the LED lamp 300 shown in FIGS. 3(a) and 3(b), an LED chip 112 mounted on a substrate 111 is covered with a resin portion 113 in which a phosphor is dispersed. A reflector 151 with a reflective surface 151a is bonded to the substrate 111 such that the reflective surface 151a of the reflector 151 is spaced apart from the side surface of the resin portion 113. Thus, the shape of the resin portion 113 can be freely designed without being restricted by the shape of the reflective surface 151a of the reflector 151. As a result, the color unevenness can be reduced significantly.
FIG. 4 shows a configuration for an LED lamp 300 in which the LED lamps having the structure shown in FIG. 3 are arranged in matrix on a substrate. In the LED lamp 300, the resin portions 113, each covering its associated LED chip 112, are arranged in columns and rows on the substrate 111, and a reflector 151, having a plurality of reflective surfaces 151a for the respective resin portions 113, is bonded onto the substrate 111.
By adopting such an arrangement, the luminous fluxes of a plurality of LED chips can be combined together. Thus, a luminous flux, comparable to that of an incandescent lamp, a fluorescent lamp or any other general illumination source that is used extensively today, can be produced easily.
Patent Document No. 5 further discloses an arrangement for reducing the color unevenness even if the phosphor is distributed non-uniformly in the resin portion 13 (i.e., if the phosphor over the upper surface of the LED chip 12 is distributed differently from that around the side surfaces of the LED chip 12). Such an arrangement will be described with reference to FIG. 5.
The resin portion 13 including the phosphor is preferably made of an epoxy resin or a silicone resin. While setting thermally, each of these resins has an extremely decreased viscosity albeit temporarily. Accordingly, if the phosphor has a mean particle size of 3 μm to 15 μm and has a greater specific gravity than the resin, then the phosphor will cause a sedimentation phenomenon while the resin is setting thermally. FIG. 5 schematically illustrates an exaggerated example in which a sediment phosphor layer 101 is formed on the bottom of the resin layer 113.
While setting thermally, the silicone resin does not have its viscosity decreased as extremely as the epoxy resin, is softer than the epoxy resin, and can relax the stress better than the epoxy resin. Patent Document No. 5 sets the conditions for matching the color of the light 103 radiated from the upper surface of the resin portion 113 with that of the light 104 radiated from the side surface of the resin portion 113 where the resin portion 113 is made of a silicone resin and discloses an arrangement for reducing the color unevenness.
Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 10-242513
Patent Document No. 2: Japanese Patent No. 2998696
Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2003-59332
Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2003-124528
Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 2004-172586
As described above, various techniques for eliminating the color evenness from LED lamps have been proposed so far. However, the present inventors discovered that the color unevenness still persists in the prior art in the following situation.
According to the conventional techniques described above, the color unevenness should have disappeared and an LED lamp that causes no color unevenness should have been realized. Actually, however, it was discovered that light rays radiated obliquely from the corners 113a of a resin portion 113 as shown in FIG. 6 produced yellowish white light. As a result, when viewed from over the substrate 111, the LED lamp radiated light in multiple colors in which central white light 103 was surrounded with yellowish white light 105, which was further surrounded with white light 104, as shown in FIG. 7.
The obliquely radiated light 105 turns yellowish mainly because the light 105 has to travel a longer distance through the phosphor than vertical light 103 or horizontal light 104. That is to say, only the light passing through the corners 113a has to be transmitted through a lot of phosphor and therefore turns yellowish. In this case, it seems to be easy to minimize the color unevenness just by rounding those corners 113a. However, the resin portion 113 is usually formed by a printing technique as shown in FIG. 8. According to such a technique, a cylindrical resin portion 113 can be formed with high precision. However, even if one tries to form such a resin portion 113 with rounded corners by the printing technique, it is difficult to cut holes in such a shape through a printing stencil. Thus, the holes cannot have the exactly intended shape. As a result, the precision decreases compared to forming the cylindrical resin portion 113.
FIG. 8 shows the process step of forming resin portions 113 by a stencil printing technique. According to this printing technique, a printing stencil 51, having a plurality of openings (or through holes) 51a in the same size and shape as the resin portions 113 to be formed, is arranged over a substrate 111, on which a plurality of LED chips 112 are arranged, such that the LED chips 12 are located within the openings 51a. Then, the printing stencil 51 and the substrate 111 are brought into close contact with each other. Thereafter, a squeeze 52 is moved in a printing direction, thereby filling the openings 51a with a resin paste 55 on the printing stencil 51 and covering the LED chips 112 with the resin paste 55. When the printing process is finished, the printing stencil 51 is removed. The phosphor is dispersed in the resin paste 60. Accordingly, when the resin paste 55 is cured, the phosphor resin portions 113 can be obtained.
Resin portions 113 were actually formed by the printing technique shown in FIG. 8 on an eight by eight array of LED chips 112. The degrees of precision and error were as follows. If the LED chips 112 had chip dimensions of 0.3 mm×0.3 mm and the resin portions 113 should have a cylindrical shape with a diameter φ of 0.8 mm, then the resin portions 113 could be formed using a stencil that had holes with a precision (or error) of about 5 μm. However, if substantially conical resin portions 113 with a trapezoidal vertical cross section should be formed to round the corners 113a, then the precision (or error) of the stencil holes reached about 10 μm. With such bad precision, even if the color unevenness that would have been caused by the corners 113a could be minimized, color unevenness would still be produced here and there due to the variation in shape between the stencil holes. As a result, the color unevenness could not be eliminated from the LED lamp as a whole.
Portion (b) of FIG. 9 is a graph showing a relationship between the color temperature [K] and the spatial distribution of light [degrees] in an LED lamp. In portion (a) of FIG. 9, this relationship is associated with the schematic view shown in FIG. 7. In portion (b) of FIG. 9, the bold curve A shows the spatial distribution of light in a situation where the color unevenness was produced. In the area 105 that looked yellowish, the color temperature decreased to about 3,000 K, for example. Naturally, such a local low color temperature area 105 should be eliminated. Thus, the color unevenness should be eliminated by minimizing the difference in color temperature as plotted by the dotted curve B.
In order to overcome the problems described above, a primary object of the present invention is to provide a method for fabricating more easily an LED lamp that causes significantly reduced color unevenness. Another object of the present invention is to provide an LED lamp that causes significantly reduced color unevenness.